A friend of mine shoots Nikon, I shoot Canon. We're always
teasing each other about it, about how the other brand is always doing things wrong. On the other hand, we also tell each other what the other brand has that we miss in our brand - I for example am pretty envious of the 200mm zoom on the new SB900. Somehow this mixture feels pleasantly fresh and light, and allows us to look at both sides of the equation.
I think it's quite impolite that she bluntly state your pictures are too dark and hers are good. Instead of trying to dodge the subject because she's rather harsh, you could turn it around and get a comparison of styles. If you print some of your photos, and let her print some of hers, you can discuss them over a nice glass of wine somewhere sunny. That gets the whole monitor calibration thing out of the way. If she says "hey, your photos look a lot darker on my monitor", you can always discuss calibration then.
Be sure to find positive things to say about her style. Keep the atmosphere bright and easy going (hence the sunny location). Perhaps by talking about your different styles, she'll be able to learn that there simply are different ways to make a photo.
I wouldn't say "my clients are happy with my shots", because that simply continues her way of discussion. If she says your shots are too dark, ask her what she means, why she thinks that. If you keep trying to get a proper reasoning out of her, perhaps it'll dawn on her that she doesn't have a real reason to say your shots are "wrong". Or perhaps she does have a very good reason - if so, you could tell her that you're glad she told you. That keeps the tone light and friendly. If she's only a bit sensitive, she'll remember it and come up with a more built up argument instead of just bashing your photos.
About having better gear: some folks make awesome shots with just a shoe box with a hole in it. Maybe you can remind her about that ;-)